Anaerobic neutrophil-dependent killing of Actinobacillus actinomycetemcomitans in relation to the bacterial leukotoxicity

The effect of leukotoxin on the interaction of Actinobacillus actinomycetem comitans with human polymorphonuclear leukocytes (PMNL) was studied under a naerobic conditions with strains able to produce high or low amounts of leu kotoxin. PMNL-morphologyl phagocytosis and killing were examined by transmi ssion electron microscopy and bioassays, respectively. At ratios of greater than or equal to 25 bacterial/PMNL, a highly toxic A. actinomycetemcomitan s strain completely destroyed the PMNL within 7 min, resulting in bacterial survival. Lowering the bacteria/PMNL-ratio enabled phagocytosis and killin g of this highly toxic strain, A. actinomycetemcomitans strains with low le ukotoxicity were effectively killed by PMNL under any condition. Presence o f specific antibodies against A. actinomycetemcomitans or of anti-leukotoxi n serum protected PMNL from being injured and allowed phagocytosis to occur . Pre-incubation of the leukotoxic strain with Porphyromonas gingivalis, a bacterium that destroys leukotoxin, abolished lysis of PMNL and inhibited p hagocytosis of A. actinomycetemcomitans. The results show that leukotoxin m ay protect of A. actinomycetemcomitans against phagocytosis by human PMNL. The protection occurs at a population level and in relation to the bacteria l load. The size of the bacterial population required to counteract phagocy tosis is dependent on the leukotoxin production of the strain.